Current internal combustion engines employed to power vehicles generally operate with air intake systems that include a throttle body and intake manifold assembly to control and direct the air flow into the engine. Intake air flow control is important not only for the obvious power output of the combustion chambers in the engine, but also for such things as minimizing noise emanating due to the air flow control system and also adequate long term operation of the system without undesirable concerns arising.
As for noise generation, this portion of the air intake system has generally been made of metal. However, in today's vehicles, an emphasis is placed on fuel economy and exhaust emissions reductions. This has lead to the desire to form the intake manifold and possibly even the throttle body out of plastic types of materials. Plastic arts  parts can be formed which are lighter in weight and can be formed into more complex shapes than equivalent metal parts, allowing for improved air flow and thus improving both fuel economy and engine performance.
Although these improvements are welcome, nonetheless, there are other characteristics of plastics that are less desirable than equivalent metal components. For example, plastic is less dense than metal, which allows for more transmission of sound through it. Consequently, sound generated within the intake manifold, for instance, will more readily pass through into the engine compartment. This sound, then, can radiate to a driver of a vehicle, who may object to it. This is particularly true since, in general, engines are being designed overall to operate more quietly, making any stray noise more noticeable.
One noise generated within the intake manifold of an engine is a whooshing noise generated by the air flow pattern created as the air flows past a butterfly or other throttle valve in the throttle body. This is particularly true for engine conditions such as tip-in or fast opening of the throttle valve. With previous engines, either due to the other background noises produced which drowned out this whooshing noise or because the intake manifold was metal which substantially dampened the noise, vehicle drivers would not hear it. Now with quieter engines and plastic manifolds, it is noticeable and objectionable to some drivers.
Another concern with the intake air flow control system is that over the long term, build up of sludge can occur around the throttle value, thus interfering with the smooth operation of this valve. One source for the contaminants causing the sludge is an exhaust gas recirculation (EGR) valve that feeds into the intake manifold just downstream of the throttle body. Due to packaging or flow reasons, it is sometimes desirable to locate the EGR valve immediately downstream of the throttle body. However, for theses designs, the EGR gasses can end up circulating back toward the downstream side of the throttle valve before flowing downstream through the intake manifold. This allows for contaminates in the EGR gasses to build up on the throttle valve as sludge over time. With a build up of sludge, the throttle valve does not move as freely and can encounter temporary sticking conditions requiring more force when a vehicle operator first presses on the accelerator pedal.
Thus, it is desirable to employ an air intake system for a vehicle engine in which the air flow is controlled such that noise generated by air flow through the throttle body and intake manifold is reduced, thus reducing the noise that a vehicle operator will hear, while also minimizing any backflow of gasses from the intake manifold into the throttle body in order to minimize the chances for sludge build up on the throttle valve.